Evolution and neurobiology of the neural circuitry underlying crawling in nudibranch molluscs

The evolution of animal behavior is, in part, a reflection of the evolution of the nervous system. In the search for general principles of how neural circuits evolve, a fundamental question is whether certain elements of nervous systems tend to be conserved whereas others are evolutionarily labile. Nudibranch molluscs are ideal for comparative studies of neural circuits because the sea slugs, as a group, show morphological and behavioral diversity yet possess a relatively simple nervous system.; The species Tritonia diomedea has been used as a model organism to understand the neural basis of various behavioral patterns, including the neural basis of mucociliary crawling. Mucociliary crawling is the primary mode of locomotion by the nudibranchs, although muscular crawling has been observed in at least two species. In addition to this behavioral variation, the nudibranchs also have variation in foot morphology. To determine whether there are aspects of the nudibranch nervous system that are correlated with behavior or morphology, the neural circuit underlying crawling was examined in a number of nudibranch species.; The central nervous system of T. diomedea has two pairs of ciliary motor neurons that produce TPeps, a group of cilio-exitatory neuropeptides. T. diomedea also has an extensive network of TPep-immuoreactive neurites at the base of the pedal epithelium. A similar network of TPep-like immunoreactivity was found at the pedal epithelium of all nudibranchs examined. This feature was also seen in several non-nudibranch species, indicating that the TPep-based cilio-excitatory system may be a general feature of the gastropods. Additionally, neurons homologous to the T. diomedea ciliary motor neurons were found in all nudibranchs examined. Differences in the size of ciliary motor neurons between species were not obviously correlated with crawling behavior or foot morphology. There was a significant correlation between the number of cells and the ratio of the length of the foot to the width of the foot. Species with a relatively wide foot tended to have more large TPep-LIR cells than species with a long, narrow foot. Changes in cell number may be a common trend in how the nervous system evolves to match morphological changes.